The present invention relates to a method for production of stainless steel.
By scrap-based production at stainless steel, steel scrap and alloying materials are melted in electric arc furnaces. The steel scrap can be both stainless steel scrap and carbon steel scrap while the alloying materials mainly consists of FeCr and nickel or nickel alloys. After melting in the electric arc furnace the steel, which has a high content of carbon, is tapped into a ladle where the slag is removed before the molten steel is transferred to a converter where it is refined in order to obtain a preset carbon content and where the final chemical analysis of the stainless steel is adjusted by the addition of additive materials before the steel is being cast.
By stainless steel it shall herein be understood steel having a chromium content of 4% by weight or more.
During melting of steel scrap and alloying materials in the electric arc furnace some oxygen is being added as a part of the raw materials and some oxygen from the surroundings is being picked up by the melt. Thus a part of the easiest oxidizable elements in the steel scrap and in the alloying materials will be oxidized and form part of the slag. One of the most valuable elements which is easily oxidized is chromium and as the slag from the steel furnaces usually is dump or placed landfills, the chromium content in the slag is lost. A high chromium content in the slag is further an environmental problem.
In order to avoid losses of chromium in the slag it is conventional practice to add silicon to the charge together with steel scrap and alloying elements during melting in the electric arc furnace. Silicon is added in the form of lumpy ferrosilicon or in the form of other silicon-rich alloys such as for instance SiCr.
Even if silicon is added in order to prevent oxidation of chromium, the slag may after melting of the charge, contain a high and varying amount of chromium oxide. This is due to a number of factors, such that the exact amount of oxygen which enters the melt in the arc furnace is not known, thus making it impossible to calculate the correct amount of silicon to be added. By a too low addition of silicon the chromium content in the slag will be unacceptably high, while a too high addition of silicon will give a too high content of silicon in the steel which is transferred to the converter and which in turn will give rise to an increased amount of slag during the refining, increased lining wear in the converter and increased refining time, which gives a lower productivity and a higher production cost. Another problem by addition of lumpy ferrosilicon together with the scrap is that one will have an inhomogeneous distribution of the supplied silicon. It may then not be sufficient time for the supplied silicon to react with chromium oxide in the slag. Sometimes this can give a slag with a very high viscosity which can make it difficult to tap the slag from the furnace such that the slag has to be remelted in the electric arc furnace during the next batch and, in addition, one will obtain a crude steel with an extremely high silicon content in the converter.
In addition to what is discussed above, it is a wish during melting of stainless steel in electric arc furnaces to use so-called foaming slag. In order to obtain foaming slag, carbon-containing materials are added to the slag whereby CO bubbles are formed in the slag by the reaction between carbon and oxides in the slag, for instance chromium oxide. The formed CO bubbles results in foaming slag. Foamed slag is more reactive than normal slag and due to its volume, it protects the furnace lining against heat from the arc. This means that the furnace can be run at a higher power that results in an increase in the melting capacity of the furnace. It has, however, been found that it can be difficult to start the slag foaming process, especially if the slag has a high content of chromium oxide.